3...Bg7 is the King's Indian Defence, one of the most heavily analysed openings in chess. Black's idea is to concede the centre to white and to then put pressure on it using the bishop on g7, often in conjunction with moves like c5 and Nc6. One of the common themes of the King's Indian Defence is for black to launch a kingside attack, typically beginning with the move f5. In reply, White will often launch their own attack on the queenside where they often have a large space advantage.

The King's Indian Defence is a very dynamic opening which can be highly risky but can also give excellent counterplay for black. As such it is often employed by players who wish for a more tactical game than is normal in Queen's pawn openings. In the late 80s and early 90s, Garry Kasparov used it regularly to devastating effect and as a result, it became very popular with players at all levels. In recent years it has been less used at the top level although Teimour Radjabov's excellent results with it helped him to win the strong Corus tournament in 2007 and possibly signalled a comeback for the dangerous defence.